This invention relates to a process for drying a gas stream which has a periodically fluctuating moisture content as well as to an apparatus for carrying out this process.
It is often required that the dew point temperature of a dry gas not exceed a predetermined limit value. When a process gas is to be treated to extract moisture in order to dry the gas, this limit value may be exceeded for a short time depending on the particular process used. For example, a process for drying plastic granules in a drying hopper using drying air is known from Graeff, U.S. Pat. No. 3,972,129. In this process, exit air emerging from the drying hopper is dried in a dryer containing an adsorption medium and then is returned again to the plastic granules as drying air. When the adsorption medium is saturated with adsorbed moisture, the medium is regenerated by treatment with hot air. During the next adsorption cycle following the regeneration, the dew point temperature in the drying air supplied by the dryer briefly rises each time a freshly regenerated, still hot drying vessel is reconnected to the adsorption circuit. Although this increase of the dew point temperature lasts for only a few minutes, it exceeds the predetermined maximum permissible limit of the dew point temperature by up to 30.degree. or 40.degree. Kelvin. When the added drying vessel has sufficiently cooled, the dew point temperature will correspondingly fall to the desired dew point or below. As a result, the current of drying air which leaves the dryer has a fluctuating moisture content. For uses which are highly sensitive to moisture, such as drying hoppers for plastic granules, this is only acceptable if the rise above the predetermined dew point temperature limit lasts for only a sufficiently short period of time. There is a need in the art for a process for regenerating a gas drying medium which can consistently maintain a dew point temperature in the gas stream below a predetermined limit.